Activation of the PO cascade is necessary for the melanization of the capsule formed by host immune cells around a foreign body such as a parasite egg (for review see Marmaras et al, 1996). This enzymic cascade constitutes one of the PDV targets (Stoltz and Cook, 1983; Lavine and Beckage, 1995; Strand and Pech, 1995b; Doucet and Cusson, 1996). For example, injection of CsIV in H. virescens larvae induces a significant reduction of the activities of several enzymes of the PO system (phenoloxidase, dopachrome tautomerase, DOPA decarboxylase) and a reduction in the concentrations of several important immune-related molecules: dihydroxy-phenylalanine, N-acetyl dopamine, and precursors of reactive quinines (Shelby and Webb, 1999; Shelby et al, 2000).
In addition, some PDVs have been shown to interfere with the synthesis of some antimicrobial compounds, such as cecropin and lysozyme (Shelby et al., 1998), haemolymphatic phospholipase C (Shelby and Webb, 1999), attacin, lectins, or serine proteases (Gillespie et al., 1997; Faye and Kanost, 1998). Whether the alteration of the expression of these compounds is directly advantageous to the parasitoid or constitutes a side effect of suppression of the host's immune response is not clearly established. It might be considered disadvantageous for the parasitoid to suppress the natural protections of its hosts against microbial agents, thus exposing the parasitized insect to infections that would kill the host before parasitoid development is completed. However, some well-characterized antimicrobial peptides, such as attacin, can also display antiparasitic properties against metazoan parasites of insects (Hu and Aksoy, 2005). The possibility that attacin or other antimicrobial peptides could have a toxic effect against parasitoid eggs and larvae would explain that the pathways controlling their production are targeted by PDVs. Parasitoids could counterbalance the suppression of their host's antimicrobial defences by producing their own antimicrobial substances. Such antibacterial and antifungal agents have been identified from adult and larval stages of parasitoids, associated or not with PDVs (Willers et al, 1982; Führer and Willers, 1986; Dani et al, 2003). In addition the temporal pattern of PDV gene expression could limit the duration of suppression of host immune responses.
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